EA Tremolo without Volume Control (and other questions)

[Transmogrifox]

An approximation that can help is all of these 3-terminal devices (MOSFETs, JFETs, BJT, IGBT, Triode) work the same way in a high-level behavioral sense:  You have one terminal connected to a higher potential and another connected to a lower potential.  A third terminal changes the amount of current that flows through the other 2 terminals.

A conversion of control voltage to current is termed "transconductance".  The details are more complicated, but as long as the device is relatively high gain they all behave nearly the same in a gain circuit such as found in the EA trem.

The thing that simplifies this and makes these devices interchangeable in a functional sense is the use of feedback.  When transconductance (device gain) is high compared to the resistor in the emitter/source/cathode to ground, then you get a voltage gain very near to unity (gain of 1) at the emitter/source/cathode.  And for clarity, this is a feedback configuration.  By increasing voltage on the gate, the current wants to increase...but..because current increases it makes the voltage on the emitter increase which wants to shut itself off and restrict current.  It balances itself at a tightly controlled constant Vbe (or Vgs) relationship that stays about the same for small fluctuations so that the emitter (source) has to follow the base (gate) voltage fluctuation at a nearly 1:1 ratio.

By this principle if you apply 10mVpp at the FET gate (for example), you get something very near to 10mVpp at the source.   The resistor in the source will conduct current in proportion to the voltage applied from the gate.  Since in a FET current can't come from the gate, there is only one place for the current to come from:  the drain.  In a BJT this relationship still holds approximately true because it will draw a much higher ratio of current from the collector than from the base (by a factor of hFE, or "beta").

So if you have, say, 1.2k in the source, and 10 mVpp at the source, then the AC portion of the current is 10mV/1.2k = 8.3 uApp through the source resistor. 

This 8.3 uApp has to go somewhere so it is supplied through the drain.  If you attach a 1.2k resistor in the drain, you end up with a unity gain inverting amplifier (pushing up at the source causes the current to pull DOWN at the drain).

If you add a 12k resistor in the drain, you get 12k*8.3uA = 100 mV, which is a gain of 10.  As you can see, this is the same as 12k/1.2k.

As a general rule you can approximate gain as Rdrain/Rsource, or Rcollector/Remitter, or Rplate/Rcathode as long as that resistor is decently large (>1k for most transistors).  I'll add in the case of tubes, though, there aren't very many gain circuits in which this assumption is useful -- for example Rcathode would have to be above 50k before this is a reasonable approximation for a higher gain triode like a 12AX7.

As the emitter/source/cathode resistor gets relatively small, then the transconductance (gain of the device) starts to have much more effect and this approximation becomes less true.

In the case of the EA tremolo, the control FET acts as a variable resistor, so the change in a device will have a stronger effect on the maximum gain (when looking into the 180 ohm series low control JFET RdsON).

That said, as long as you get the circuit biased properly for the different devices you could use a MOSFET, a Triode (tube), a JFET or a BJT and get approximately the same result.

The BJT is the most different because it is slightly different in how it works.  It takes some current on the input, so it reflects back a smaller equivalent resistance from the emitter if the emitter resistor is small.  In that sense, an EA trem with a BJT and no buffer reflects a variable resistance back to the base of the transistor, and that resistance is changing with the LFO.

[antonis]

  Well said...!!! 

Just a note to:
<As a general rule you can approximate gain as Rdrain/Rsource, or Rcollector/Remitter, or Rplate/Rcathode as long as that resistor is decently large (>1k for most transistors)>

If there is a negative feedback loop (which is true in most cases..) the Gain is proportional to Rfeedback/Rbase, as long as the ratio of Rf/Rb is much lower than Rcollector/Rbase and Rf is much higher than Rc..
(same for Rdrain & Rgate..)

[Gargaman]

Thanks again!
I'm trying to follow everything been discussed by now!
I think there's a lot of information that's gone be very useful, for me, at least in the future..
Seems like I'm not on that level yet, so I think it's better understanding the basics a bit more about a single BJT and how it works, in boosters and simple drives, like Electra Distortion, for example.
I need to understand how to bias a transistor properly to get a 1:1 gain first, and then go into clipping territory. Voltage gain and current gain also are situations not clear for me.
Altought I might be going away of the initial subject of the post I would like to ask two or more things about how the audio signal (AC signal) relates with the DC signal while entering a simple transistor circuit.
Thinking about a circuit like LPB Boost (no distortion for now):
After the input cap (that blocks DC current), the signal coming from the input relates (mixes, add, what happens?) with the DC coming from 9V power supply to ground. Then the magic happens around the transistor and resistors along it. Finally, the signal goes through the output cap (that also blocks DC current).
I've read that the small flutuactions (voltage? current?) in the base causes big flutuations (voltage? current?) on the collector, but still don't understand what happens around and how the audio signal goes from in to out after that interaction with DC.

It's hard to formulate my doubts! Looks like I got dozens of pieces of a puzzle, I can glimpse the final result, but still can't arrange everything together.

[antonis]

the signal coming from the input relates (mixes, add, what happens?) with the DC coming from 9V power supply to ground.

Better to read few things about BJT as a current source..
(before some guys come and slap your tonge with a rule (or similar..)   

quickie: You may substitute "mixes, add, what happens" with "disturbance" 

Signal goes to transistor base and is further "lost" via emmiter to ground - no original signal anymore..!!

It have annoyed enough Collector's serenity so Collector reacts in a rude manner, resulting in an exravagant punishment..

Emitter is busy to maintain a voltage difference accepted by Base so it can pass the original signal but Emitter's resistor doesn't like the whole situation so it raises Emitter's voltage to anticipate Collector's rudeness..

There is a no man's land between Collector and Emitter which plays the role of boxing bug..


Sorry guys but it's quite late here..

[Transmogrifox]

In some post recently PRR gave a good analogy about flooding your basement with a 4" pipe and draining it with a 4" pipe, controlling the drain with a flap and watching the water level in your basement go up and down.  I thought is was a good analogy -- worth a read if you find it.  Don't search more than a month back.

It's not too far removed from the idea of sound in air.  Atmospheric pressure is nearly constant at a certain altitude...changes very slowly compared to sound waves.  This could be called the "DC" air pressure.

When sound waves travel through the air, they make small fluctuations in the air pressure which makes your ear drums move back and forth.  An audible sound wave would be the "AC" part.

Your speakers rest in a center position.  When electric energy is applied to the coil, it makes them move forward and backward.  If you had your speaker forced as far forward as it can go, then an electric signal pushing it forward would have no effect (except it might blow up your amplifier).  The reverse moving electric signal would pull the speaker backward, then it would return to full forward.  This would sound horrible.

The speaker being pushed full forward can be compared to applying an AC signal to a BJT with its base grounded (not biased).  In that state the collector voltage is stuck high and can't go any higher.  When you give some voltage at the base, it begins to pull down and causes the voltage at the collector to move down.

Bias is to make the collector voltage stay somewhere near 1/2 the supply voltage so there's room for it to move up and down. 

The wave doesn't move through the transistor.  Its movement is only copied (mirrored) at the collector...and that's what these circuits do.  It's like, if the guy across the room moves his hand up 1 foot, I move my hand down 3 feet.  The guy watching me through the window in the next room moves his hand up 2 feet when I move mine down 1 foot, etc.

The electrons in your guitar pickup don't touch the speaker coil any more than the guy in the other room ever touches my hand.  It's just a big game of copy-cat from one transistor circuit to the next and we get distortion when the first guy moves his hand 1 foot and the 3rd guy can't move his hand up 6 feet -- it stops as high as he can reach at about 2 feet above his shoulder.

So, to bias his hand at 1/2 the supply would be to hold it straight out parallel to the ground so he has room to move it up 2 feet and down 2 feet.

Maybe that helps.  Something maybe easier to follow than the more technical stuff.